In an optical pickup used in an optical disk device, etc., in the case where an objective lens is moved in the tracking direction, a light spot is moved on the surface of a photosensitive element, which gives rise to off-set errors in the focusing error detection signal. In order to avoid it, various sorts of optical systems have been proposed. FIG. 4 shows various sorts of such prior art optical pickup optical systems
In FIG. 4, reference numeral 10 indicates the center line, around which the optical disk is rotated, and 20 represents a line in the radial direction on the disk. In the optical system indicated in 4(1) a beam splitter 12 is used instead of a prism and astigmatism is produced by a cylindrical lens 14. Reference numeral 11 is a laser light source; 13 is a minor for bending a light beam coming from the beam splitter at a right angle to lead it to a recording track on the disk; 15 is a convex lens for increasing the magnification for the received light; and 16 is a photosensitive element. An objective lens is disposed between the mirror 13 and the disk.
The optical system indicated in FIG. 4(2) is an optical system, in which a half mirror 22 is used as the beam splitter and a new astigmatism is synthesized by means of a cylindrical lens 24. Reference numeral 21 is a laser light source; 23 is a mirror for bending a light beam coming from the beam splitter 22 at a right angle to lead it to a recording track on the disk; 25 is a convex lens for increasing the magnification for the received light; and 26 is a photosensitive element. An objective lens is disposed between the mirror 23 and the disk.
The optical system indicated in FIG. 4(3) is an optical system, in which a half mirror 32 is used as the beam splitter and the optical axis connecting 32 is used as the beam splitter and the optical axis connecting this beam splitter 32 with a photosensitive element 36 is inclined by several tens of degrees (e.g. 45) with respect to the objective lens drive direction for the tracking. The beam splitter 32 constitutes astigmatism generating means. Reference numeral 31 is a laser light source; 23 is a mirror for bending a light beam coming from the beam splitter 32 at a right angle to lead it to a recording track on the disk; 35 is a convex lens for increasing the magnification for the received light; and 36 is a photosensitive element. An objective lens is disposed between the mirror 33 and the disk.
In any one of the optical systems described above, in the case where the objective lens is moved in the tracking direction, when a light spot is moved not along two dividing lenses, which divide the photosensitive surface of the photosensitive element into four sections and are perpendicular to each other, as indicated by the broken line in FIG. 5(2), off-set errors are produced in the focusing detection signal. Therefore the astigmatism generating direction is rotated by several tens of degrees with respect to the objective lens drive direction for the tracking so that the light spot is moved along one of the driving lines on the four-divided photosensitive element, as indicated in FIG. 5(1), in order to reduce the off-set errors described above.
According to the optical system indicated in FIG. 4(1), it is necessary to use an expensive prism type beam splitter, which causes to raise the cost of the optical pickup. According to the optical system indicated in FIG. 4(2), since the astigmatism is produced by combining the beam splitter with the cylindrical lens, it is not possible to select freely the production direction and the magnitude of the astigmatism and therefore it has a problematical point that design freedom is low. According to the optical system indicated in FIG. 4(3), it is necessary to bend complicatedly the optical axis. Therefore it is difficult to secure the production precision, which is apt to cause lowing in the performance. Further, according to the optical systems indicated in FIGS. 4(2) and 4(3), since focusing errors are produced by the astigmatism of the beam splitter, in order to obtain predetermined focusing error detection characteristics, the magnitude of the astigmatism (range of astigmatism), i.e. thickness and material of the beam splitter are restricted. In addition, in any one of the optical systems, another member such as a convex lens is necessary for obtaining a predetermined value for the magnification on the light receiving side.